Suppression of interference



July 2, 1940 W. R. KOCH SUPPRESSION 0F INTERFERENCE Filed Dec. .30 1937 ZSmaentor .11'

Patented July 2, 1940 SUPPRESSION OF INTERFERENCE Winfield a. Koch, Haddon field, N. .L, assignor to Radio Corporation of America, a corporation of Delaware Application December 30, 1937, Serial No. 182,438

15 Claims.

This invention relates to the suppression of interference in coaxial cable circuits such as are sometimes used in television systems and has for its principal object the provision of an improved system and method of operation whereby the low frequency interference produced in such circuits by ground currents or the like is avoided.

In the use of coaxial cable circuits for the transmission of pictures, difficulty is sometimes encountered due to low frequency currents which are induced into the outer conductor of the cable and combined with the video signal transmitted over the cable to prevent satisfactory synchronization of the picture. It has been found that this interference may be reduced to a negligible value by grounding'the outer conductor at only one point, such as at either the sending or receiving end. Such grounding, however, has the disadvantage that it prohibits the grounding 20 of the power supply systems utilized to provide the plate and cathode heating voltages for the amplifiers connected to the ungrounded end of the coaxial cable. grounded directly, at the transmitting end of the coaxial cable, grounding of the receiver end of the outer cable conductor through a resistor of the order of one or two hundred ohms great- 1y reduces the ground currents flowing through ruary 23, 1938, and assigned to the same as-- signee as theppresent application. In this case, however, this voltage appears in the amplifier output circuit and if the amplifier is of the screen, grid type, the injection of this interference voltage into the output circuit may not produce serious difiiculty. The use of other types of amplifiers makes especially desirablethe provision of some means for neutralizing the effect of the interference voltage in the amplifier plate circuit.

As distinguished from the invention of the aforesaid application, the present invention con templates the introduction of a part of the interference voltage into the amplifier input cir- With the outer, conductor cuit for the purpose of counterbalancing the interference voltage appearing in the screen grid and/or output circuits of the amplifier. In one form of the invention, the resistor across which the interference voltage appears is provided with an adjustable tapping terminal whereby the amplifier input circuit and the amplifier output circuit components of the interference voltages may be balanced one against the other so as to neutralize their effect on subsequent stages of .the amplifier. In another form of the invention, this result is accomplished-through a degenerative amplifier interposed between the main amplifier input and output circuits.

The invention will be better understood from the following description considered in connection with the accompanying drawing and its scope is indicated by the appended claims.

Referring to the drawing,

Figures 1 and 2 illustrate a video signal channel wherein the amplifier connected at the re-' ceiver end of a coaxial cable is provided with an impedance device such as a resistor arranged to localize the interference voltage, and

Figure 3 illustrates the use of a degenerative circuit between the amplifier input and output circuits for balancing this interference voltage out of the subsequent stages of the amplifier.

The system of Fig.1 includes an input amplifier I0 which has-its output terminals II and I2 connected respectively to the inner conductor -l3 and outer conductor M of acoaxial cable. It will be noted that the outer conductor M is ,connected directly to ground through the lead l2.

At its opposite end, the coaxial cable l3l4 is connected to a resistor I5 which is coupled through a capacitor l6 tothe input circuit of an amplifier H, the usual grid leak resistor and source of bias potential being provided in the input circuit of. this amplifier. It will be noted that the outer conductor I4 is connected to ground through "a resistor l8 and that a part of this resistor'is included in the input circuit of the amplifier H. In order to exclude interference voltages from the output circuit, the oath ode of the amplifier ll and the low potential side of the amplifier control grid are grounded through a predetermined part of the resistor E8. The output circuit of the amplifier ll includes a resistor [9, a resistor Hand a reactor 2! which is connected in shunt to the resistor 20 for maintaining a desired frequency characteristic and phase relation in the output circuit. Plate potential is derived from a potential source 22. Positive potential is applied to the screen grid of the amplifier I! through a resistor 23, an altemating current by-pass capacitor 24 being connected between the screen grid and the cathode.

The output circuit of the amplifier I1 is coupled to the input circuit of an amplifier 25 through a capacitor 26 and video output current is derived from the plate circuit of this amplifier.

Since the resistor I8 is connected partly in the output circuit and partly in the input circuit of the amplifier H, the interference voltage, of course, appears in both these circuits. By suitable adjustment of the intermediate terminal of the resistor, the interference voltage component in the input circuit, when amplified by the tube and thus made to appear in the anode circuit, will balance the interfercnce voltage directly introduced in the anode-cathode path and thus neutralize the interference currents through the output impedance I9, 20, 2!. In this connection, it should be noted that the by-pass capacitor 24 presents negligible impedance to the interference currents and that the resistor presents a very substantial impedance to these currents, so that no interference voltage will be present between the screen grid and cathode of amplifier H.

The modification of Fig. 2 is in most respects similar to that of Fig. 1, but differs therefrom in that the capacitor 24 and the resistor 23 are omitted. .The interference voltage existing across the lower portion of the resistor I8 will, therefore, be directly impressed between the cathode and the screen grid of the amplifier tube ll,

' this circuit.

and through the effect of this screen grid voltage on the plate current, tend to produce interference currents through resistor I9. By adjusting the tapping point on resistor l8, the portion of the interference voltage applied between control grid and cathode can be adjusted so as to just balance out the interference currents in resistor I9 introduced both directly and through the screen-grid voltage variations.

The modification of Fig, 3 difiers from those of the previous figures in that an auxiliary amplifier 21 is connected in parallel with the amplifier H for neutralizing the interference voltages in the input and output circuits of these amplifiers. To this end, the high potential sides of these input and output circuits are interconnected through a resistor 28 and a capacitor 29 upon which are impressed both signal and interference voltages. The resistor 28 is provided with a terminal 30 which isadjusted to a point where the signal potential is zero and there is impressed on the control grid 3| of the amplifier 21 a potential dependent on the interference voltage which is amplified by the amplifier 21 and applied to the output circuit of the amplifier I'I tending to reduce the interference voltage in It will be noted that a suitable positive potential is applied to the screen grid 32 of the amplifier 21 through a resistor 33.

I claim as my invention;

l. The combination of a resistor, a coaxial cable including an inner conductor and an outer conductor, said outer conductor being grounded at one point and also connected to ground through said resistor at another remote point, means for applying signal impulses to said cable, and means including an amplifier provided with an input circuit including one part of said resistor and with an output circuit including an other part of said resistor.

2. The combination of an impedance device, a

coaxial cable including an inner conductor and an outer conductor, said outer conductor being grounded at one point and also connected to ground through said device at another point,

means including an amplifier provided with aninput circuit including one part of said device and with an output circuit including another part of said device, means for applying signal impulses to said cable, and means for adjusting the relative impedance of said parts.

3. The combination of a coaxial cable including an inner conductor and an outer conductorwhieh is grounded at one point, a resistor con- .necting said outer conductor to ground at aning into said output circuit a voltage whereby.

said interference voltage is neutralized.

4. The combination of a resistor, a coaxial cable includlng an inner conductor and an outer conductor which is grounded at one point and also connected to ground through said resistor at another point, means for applyingsignal im-' pulses to said cable, and means for neutralizing noise representing voltages developed across said resistor, and means includingan amplifier provided with an input circuit including one part of said resistor and with an output circuit in- .eluding a different part of said resistor.

5. The combination of a resistor, a coaxial cable including an inner conductor and an outer conductor which is grounded at one point and also connected to ground through said resistor at another point, means for applying signal impulses .to said cable, and means including an amplifier provided with an input circuit including in series one part of said resistor and with an output circuit including in series another part of said reslstor, said amplifier having a cathode connected to said resistor at a point intermediate said outer conductor and ground.

6. The combination including a transmission lme'having a pair of conductors, one conductor of said pair being connected to ground at one conductor of said pair and a point intermediate the ends of said impedance, and said output circuit including that part of said impedance between ground and said intermediate point.

'7. The combination including a coaxial transmission line having an inner conductor and an outer conductor, said outer conductor being connected to ground, at one point and also connected to ground through an impedance at another remote point, means for applying signal impulses to said line, and an amplifier having input and output circuits, said input circuit bemg connected between said inner conductor and a point intermediate the ends of said impedance, and said output circuit including that part of said impedance between ground and said intermediate point.

8. The combination including a transmission line having a pair of conductors, one conductor of said pair being connected to ground at one point and also connected to ground through a resistor at another remote point, means for applying 7 signal impulses to said line at said one point, and an amplifier at said remote point having input and output circuits, said input circuit being connected between the other conductor of said pair and a point intermediate the ends of said resistor, said output circuit including that part of said resistor between said intermediate point and ground, said intermediate point being selected so that noise-representing currents flowing through said resistor from said one conductor impress voltages on said amplifier input circuit which oppose the similar voltages induced in said amplifier output circuit by noise-representing currents flowing through that part of said resistor between said intermediate point and ground.

9. The combination, with a transmission line subject to interference currents and having one conductor connected effectively to ground at at least one point, "of an amplifier having input and output circuits, means included in said ground connection and in common circuit with said amplifier input and output circuits for applying noise-representing voltages to said input and output circuits, the relative amplitudes of said voltages being so selected that noise-representing currents in said output circuit are minimized.

10. The combination, with a two-conductor transmission line subject to interference voltage,

other part ofsaid impedance means in said output circuit.

, 11. The invention as set forth in claim wherein said impedance means is a resistor potentiometer for varying the ratio between said parts. A

12. The combination, with a two-conductor.

transmission line disposed to interference currents, of amplifying means having an input circuit connected to said line for receiving transmitted signals and being subject to said interference, impedance means connected between one of said conductors and ground for developing across said impedance means a voltage drop from said interference currents, and means for deriving from said impedance means a selected inter- 'ference potential for neutralizing the effect of said interference on said amplifying means.

13. The invention as set forth in claim 12 wherein said amplifying means is a multielec-v trode electron discharge device having one electrode connected effectively to one point on said impedance means and another electrode connected to another point on said impedance means.

14. The combination, with a two-conductor transmission line disposed to interference currents, of means for eifectively grounding one of said conductors at one location, means for applying signal potentials to said conductors at said location, impedance means connected between said one of said conductors and ground at another location for developing a voltage drop from said interference currents, amplifying means connected to said line for receiving signals transmitted over said line and subject to said interference currents and having input and output circuits, and means for deriving from said impedance means a selected part of said voltage and impressing said selected voltage on said input circuit and means for impressing another selected part of said voltage on said output circuit. 

